The field of display device is very important for the information and communication industry. Recently in accordance with the development of information and communication technology, more advanced performance in this field has been asked for. Display can be divided into luminescent type and non-luminescent type. The luminescent type of display comprises Cathode Ray Tube (CRT), Electroluminescence Display (ELD), Light Emitting Diode (LED), Plasma Display Panel (PDP), etc. The non-luminescent type of display comprises Liquid Crystal Display (LCD), etc.
The luminescent and non-luminescent type of displays have such basic performances as operation voltage, consumption power, brightness, contrast, response rate, life time, etc. However, LCD, which has been widely used up to now, has some problems in the above basic performances in regard to response rate, contrast, and sight dependency. Thus, the LED-using display is anticipated to take the place of next-generation display device by solving the above LCD problems and by providing many other advantages such as fast response speed, no need for back light due to self-emission, and excellent brightness.
However, LED is mainly used with a crystal form of inorganic material, and so is hard to be applied to a large size of electroluminescent device. In addition, the electroluminescent device using inorganic material is very expensive and needs more than 200 V of operation voltage. However, Eastman Kodak reported in 1987 that the company manufactured a device made of a material having π-conjugate structure such as alumina quinine, and thereafter, the study for electroluminescent device using organic material has been more active.
The electroluminescence device (EL device, below) can be divided into inorganic EL device and organic EL device, depending on what material is used to form the emission layer (emitter layer).
The organic EL device, which is a self-emitting type of device that electrically excites fluorescent organic compound, is superior to the inorganic EL device in brightness, operation voltage, and response rate, and also can emit multi-colors.
In addition, the organic EL device is a luminescent device to emit in low voltage current, and has superior properties such as enhanced brightness, high speed of response, wide viewing angle, plane luminescence, slim type, and multi-color luminescence.
Thus, the organic EL device is expected to be applicable to a full-color plat panel display due to such superior properties that cannot be found in other displays.
C. W. Tang et al. reported the first practical device performance of the organic EL device in Applied Physics Letters, vol. 51 (12) pp 913-915 (1987). They developed a laminated-structure thin film (a hole transport layer) of formed by diamine analogues as organic layer and a thin film (an electron transport layer) formed by tris(8-quinolinolate) aluminum (Alq3, below). The laminated structure can lower the injection barrier of electron and hole from both electrodes to the organic layer, and also can enhance the re-combination probability of electron and hole from the inner organic layer.
Later, C. Adachi et al. developed an organic EL device having an organic luminescent layer with three-laminated structure of hole transport layer, emission layer, and electron transport layer [Japanese Journal of Applied Physics, vol. 27 (2), pp L269-L271 (1988)], and two-laminated structure of hole transportable emission layer and electron transport layer [Applied Physics Letter, vol. 55 (15), pp 1489-1491 (1989)], and showed that the optimization of device property can be achieved by constructing a multi-layer structure suitable for materials and combination thereof.
The organic EL comprises a first electrode (anode), a second electrode (cathode), and organic luminescent media. The organic luminescent media have at least two separate organic luminescent layers, i.e. one layer to inject and transport electron, and the other layer to inject and transport hole into the device. In addition, another multi-layer of thin organic film can be involved. The above layers to inject and transport electron and hole each can be divided into an electron injection layer, an electron transport layer, a hole injection layer, and a hole transport layer. In addition, the organic luminescent media can further include an emission layer besides the above layers.
The simple structure of organic EL device comprises a first electrode/an electron transport layer, and an emission layer/a second electrode. In addition, the structure of organic EL device can be separated into a first electrode/a hole injection layer/a hole transport layer/an emission layer/an electron transport layer/an electron injection layer/a second electrode.
The operation principle of the organic EL device having the above structure is as follows.
If the voltage is applied to the anode and cathode, the hole injected from the anode is transferred to the emission layer via the hole transport layer. Meanwhile, the electron is injected from the cathode to the emission layer via the electron transport layer. The hole and electron are re-combined in the emission layer to form exiton. The exiton is changed from the excitation state to the basic state, and thereby the fluorescent molecule of the emission layer becomes luminescent to form images.
The manufacturing process of a conventional organic EL device is explained with referring to FIG. 1 as follows.
First of all, an anode material 2 is formed on a transparent substrate such as glass 1. This time, Indium Tin Oxide (ITO: In2O3+SnO2) is generally used as the anode material 2. On the anode material 2 may be formed either the hole injection layer (HIL) 3 or the hole transport layer (HTL) 4, or both HIL 3 and HTL 4 in order.
Here, Copper (II) Phthalocyanine (CuPC) of the thickness of 0 to 30 nm is generally used as HIL 3, and N,N-di(naphthalen-1-yl)-N,N′-diphenylbenzidine (NPD) of the thickness of about 30 to 60 nm as HTL 4.
Then, an organic emission layer 5 is formed on HIL 3 or HTL 4. Particularly, the luminescent material may be used alone as the emission layer 5, or used by doping a small quantity of impurity to the host material as occasion arises. For example, in case of green color emitting, tris(8-hydroxyquinolate) aluminum [Alq3] of the thickness of about 30 to 60 nm is usually used as host, and MQD (N-methylquinacridone) is usually used as dopant of the organic emission layer 5.
Next, an electron transport layer 6 or an electron injection layer 7 is independently or subsequently formed on the emission layer 5. Alq3 is usually used as the electron transport layer in the thickness of about 20 to 50 nm, and alkali metal analogue is used as the electron injection layer in the thickness of about 30 to 50 nm. In case of green color emitting, since Alq3 used as the organic emission layer has superior electron transport ability, the electron transport layer 6 or the electron injection layer 7 cannot be used.
Further, a second electrode (cathode) 8 is formed on the electron transport layer 6 or the electron injection layer 7, and a protecting layer is formed at the last.
Three luminescent devices emitting green, red and blue colors usually need to actualize full color of the organic EL device.
The blue color is actualized by doping a blue color emitting dopant onto a blue color emitting host and using Alq3 as the electron transport layer, and Alq3 may be omitted depending on the property of blue host. In case of red color emitting device, the red wavelength can be obtained by doping a red color emitting dopant, instead of a green color emitting dopant, during the above preparation of the device.
In case of green color emitting device, Coumarine 6 or Quinacridone analogue is used as dopant, and in case of red color emitting device, DCM (4-dicyanomethylene-6-(p-dimethylaminostylyl)-2-methyl-4H-pyrnae) analogue, such as DCM1 or DCM2, etc., is used as dopant [see, Journal of Applied Physics, 3610 (1989)].
However, in case of green color emitting device, the safety of the device is evaluated to have reached a practical level, but the red color emitting device has a problem that the luminescent color and the safety of the device have not reached such a level.
That is, among three luminescent devices of red/green/blue, the development of red color emitting device has been latest, and sufficient brightness and chromaticity therefor could not be obtained yet. For example, the peak wavelength of luminescent spectrum of the above DCM is about 600 nm, and the half band width is as broad as about 100 nm, therefore, the chromaticity as red corresponding to full-color is greatly lowered. In addition, if the concentration of red color emitting dopant such as DCM is small, the orange region spectrum is obtained, and if that concentration is rich, the red region is emitting but the luminescent efficiency is lowered by self quenching. Further, the red color emitting device using Alq3 [tris(8-hydroxy quinolate)aluminum] doped by DCJTB [4-(Dicyanomethylene)-2-tert-butyl-6-(tetramethyljulolidi-4-yl)-4H-pyran] as electron transport material is not satisfactory as display material in view of lightness and chromaticity thereof.
However, the organic metal complexes, whose central metal is europium, have been known as the red color emitting device having high chromaticity, but the maximum lightness of the organic EL device using them is very low [see, Applied Physics Letter, 65 (17), 2124˜2126 (1994)].
In addition, Japanese Patent Publication No. 1999-329731 has been disclosed to manufacture the red color emitting organic EL device having high brightness by using specific di-styryl compounds, but the half band width of luminescent spectrum is more than 100 nm, and thus the chromaticity thereof cannot be said complete.
To solve the above problems, the present inventors have conducted intensive studies to develop a red color emitting material that is safe at high brightness and has good chromaticity, and completed the present invention.